The prior art discloses machine tools in which, when a workpiece is machined by a tool, the rotary motion of the tool can be superimposed by an ultrasonic vibration of the tool.
EP 1 763 416 B1 describes in this connection a tool including a tool holder which at a first end has a tool holder support for adaptation to a rotary spindle nose and at a second end opposite the first end has a tool support, and including a tool head which can be inserted in the tool support, wherein the tool holder comprises a vibration motor.
In such a machine tool, an ultrasonic transducer in the tool holder, which generates the ultrasonic vibration of the tool, a vibrating body and the tool inserted in the tool holder constitute a vibratory system which is caused to vibrate mechanically by an electrical signal, wherein the greatest possible mechanical vibration amplitude is obtained when the vibratory system is stimulated with its resonance frequency.
In so doing, the problem arises that the resonance frequency can change in the course of a processing operation. There are substantially three reasons for this. On the one hand, the vibratory system or parts thereof can heat up during processing, thus changing the properties of the material. This leads to a temperature drift of the resonance frequency.
On the other hand, the vibration is damped by the processing forces when the tool contacts the workpiece to be processed, wherein in the case of a damped vibration the resonance frequency of the system is lower than the free resonance frequency of the system.
Furthermore, a new, coupled vibration system is formed by coupling the vibration system to the workpiece, the resonance frequency of said vibration system usually being higher than the free resonance frequency. In practice, these three effects occur in combination, and it depends on the specific processing situation which effect dominates.
It should also be noted that in addition to the displacement of the resonance frequency a change in the power also plays a part since due to the interaction between the tool and the workpiece a higher output voltage may be necessary to achieve the same power.
If the free resonance frequency is used for the stimulation and the actual resonance frequency of the system differs therefrom during processing, the vibration of the tool will have a lower amplitude, thus rendering the processing less efficient.
For this reason it is important to detect a change in the resonance frequency of the vibratory system to be able to correspondingly adapt the vibration parameters in such a way that the largest possible vibration amplitude is obtained again.
It is known from ultrasonic welding applications to determine for this purpose both the free resonance frequency and a change in the resonance frequency of the system from initial values of the generator which supplies the electrical signal for the mechanical vibration to the piezo drive in the tool holder. The generator sees that the vibratory system connected via an inductive transmission path has an electrical impedance which depends on the frequency and has a minimum at the mechanical resonance frequency. Accordingly, the generator readjusts its frequency in the case of a shift of the resonance frequency until it reaches the impedance minimum again. In addition to the frequency of the impedance minimum, the impedance value as such also changes due to the processing operation, i.e. a higher output voltage is necessary to drive the same power.
However, this method is not suitable for machining because, unlike with ultrasonic welding, the impedance curves of the employed sonotrodes are much more complex with inserted tools. On the one hand, there are significantly more impedance minima due to the many different vibration modes of the tools with complex forms. On the other hand, the influencing variables which cause a shift of the resonance frequency have a more extreme effect, i.e. the frequency shift can be so large that further impedance minima are skipped. A sonotrode exerts almost the same pressure on the workpiece during the entire welding process. This results in a single frequency shift which is the same in recurring processes and in which the impedance minimum can always be clearly identified. On the contrary, the frequency shift constantly changes during machining on account of varying advancing conditions of the tool into the material and, as described above, the assignment is often no longer possible by means of an impedance measurement alone.
This is because a great many tools with different forms are used, e.g. drills and milling cutters having different dimensions and cutting tools having different cutting geometries, which leads to a higher variance in the shape of the impedance curve compared with ultrasonic welding. Furthermore, the force acting on the vibratory system is generally significantly higher in a machining operation, and therefore the change in the impedance curve is much more marked.
In addition, on account of the recurring processing steps during welding the dominating frequency shift effect can be well predicted, which limits the possible reactions of the system. On the contrary, all effects have to be taken into consideration in a machining operation, and this is why the prediction possibilities and/or the possibilities for limiting the control parameters are insufficient.
Moreover, it is not possible to distinguish bending vibrations or the like from axial vibration modes only on the basis of the impedance measurement. There are also purely electrical resonances which do not produce any vibrations at all. These parasitic effects cannot be detected by the known methods.
A further problem arising when monitoring the vibration on the basis of the generator output is that it is not known which part of the power is actually incorporated into the vibration generation and which part goes into other processes such as the heating of the components involved. Therefore, it is possible that changes in the vibration are not detected because, although the part of the power which is provided by the generator and is used for the vibration generation is subject to change, the overall power provided by the generator does not change.